1. Objective
The present invention relates to the reduction in ESR obtained by infusing the carbon layer of a solid electrolytic capacitor with an intrinsically conductive polymer.
2. Prior Art
Miniaturization of electronic devices is an important and well established trend in the electronics industry, particularly in the field of computers and telecommunications. Valve metals such as tantalum, aluminum, and niobium are particularly suited for the manufacture of high surface area solid electrolytic capacitors in which the valve metal serves as the anode, and an oxide of the valve metal, formed by anodic oxidation of the valve metal surfaces, serves as the dielectric. In order to maximize the dielectric surface area, and hence increase the volumetric efficiency of the capacitor, the valve metal substrates typically are porous bodies.
Digitization of electronic circuit design has increased the demand for capacitors with high capacitance and low ESR (equivalent series resistance) in the high frequency region. The ability of capacitors to perform their filtering function is limited by the unwanted xe2x80x9cparasiticxe2x80x9d resistances that result from the use of real-world, non-ideal materials in their construction. These parasitic resistances, collectively known as the finished capacitor""s equivalent series resistance (ESR), manifest themselves in the user""s electrical circuit as though they were a discrete resistor connected in series with the capacitance of the capacitor. Whereas ideal capacitor elements inherently oppose rapid voltage shifts in the face of changing current, the voltage across resistors changes instantaneously and proportionally to changing current. As a result if a practical capacitor has significant ESR, the resulting instantaneous voltage shifts across the capacitor""s ESR, that are proportional to changes in circuit current, undermine the voltage-stabilizing influence of the capacitor. If the ESR becomes too large, the capacitor becomes useless as a filtering device. Traditional solid tantalum, aluminum and niobium capacitors employ manganese dioxide as the primary component of the cathode. Wet aluminum electrolytic capacitors employ a wet electrolyte in the cathode construction.
Early conductive polymers attempted to mimic the properties of metals by using metal or carbon flakes or fibers, with uneven results. Intrinsically conductive polymers were discovered at the University of Pennsylvania in the mid-1970""s and the discoveries won the Nobel Prize in 2001. At present, the polymers such as polythiophenes, polypyrroles, polyanilines, polyacetylenes, polydiacetylenes, polynaphthalenes, and their derivatives are finding increased utilization as the primary component of the cathode in solid electrolytic capacitors manufactured from valve metals due in large part to the high conductivity of these materials relative to the more traditional cathode materials. The higher conductivity of intrinsically conductive polymers results in substantial reductions in ESR relative to traditional cathode materials. Low ESR and high capacitance reduces the number of components required to achieve design solutions, resulting in a reduction in board space, as well as reduced cost.
The electronics industry uses automated equipment to place components on a board prior to a solder reflow process which secures the component to the board. The so called xe2x80x98pick and placexe2x80x99 process uses a vacuum chuck to pick the component up and place it in the proper location on a circuit board. The top surface of the component must be flat and smooth in order for the vacuum chuck to reliably manipulate the component. Manufacturers generally encapsulate components using a transfer molding process to provide this flat smooth surface. The transfer molding process subjects the component to therno-mechanical stress.
Following the placement of the component on a circuit board a solder reflow process is generally used to secure the component to the board. During this operation the component is subjected to temperatures above the melting point of solder, in most cases the peak temperature of the solder reflow process exceeds 200xc2x0 C. and maybe as high as 250xc2x0 C. This thermal excursion places thermo-mechanical stresses on the component inside the molded case. These thermo-mechanical stresses can cause an increase in the ESR of a solid electrolytic capacitor.
In order to minimize the resistance of the cathode of solid electrolytic capacitors the devices are dipped in a silver paint (xe2x80x9cdip silvexe2x80x9d), which when dried provides a highly conductive outer coating. Between the primary cathode material and silver layer a carbon layer provides improved ESR stability to therno-mechanical stresses such as encapsulation or board mounting. Although the carbon layer provides improved ESR stability, it increases the path length for conduction of current from the external circuit to the dielectric, thus increasing the device ESR. To achieve a thin carbon layer which resists ESR changes during transfer molding and reflow, the carbon is applied by dipping the device in a suspension of highly graphitized colloidal carbon. The resultant carbon layer is inherently porous, and has a higher resistance than would a fully densified carbon coating. A means of increasing the conductivity of the carbon layer is therefore desirable.
Commercially available carbon suspensions contain graphite particles, a solvent, and a resin to bind the particles together and to the underlying primary cathode material. After dipping in the carbon suspension, the carbon is dried. Although the conductivity of compacted and heated carbon, i.e., graphite, is on the order of 700 Siemens/cm, the conductivity of the dried carbon layer is typically 10-50 Siemens/cm. The lower conductivity of the dried carbon layer is due to the presence of the non-conductive binding resin and microscopic voids between the graphite particles.
Tadanobu et al., U.S. Pat. No. 6,343,005 B1 discloses and claims a solid electrolyte capacitor particularly characterized in that the negative electrode terminal includes a carbon layer to which has been added a catechol or pyrogallol derivative. The patentees teach that 0.2 to 1.2 parts by weight of the derivative per 1 part by weight of carbon particles improves the uniformity of the carbon layer by reducing surface tension of the carbon solution. As a result, contact resistance between the solid electrolyte and the carbon layer is reduced and improved ESR characteristics are obtained in high frequency ranges. No benefit related to the conductivity of the catechol or pyrogallol is asserted, the benefit residing solely in improvements to the quality of the dispersion of the carbon. No polymerization step is disclosed or suggested.
It has been found that when a conductive polymer is applied on a dielectric surface before a carbon layer is applied and the carbon layer is subsequently infused with a conductive polymer the adhesion between cathode layers is increased and the ESR of a solid electrolytic capacitor is improved both after fabrication and after board monitoring.
It is an object of this invention to improve the conductivity of a carbon layer cathode by infusing it with a conductive polymer, preferably an intrinsically conductive polymer.
It is a further object of this invention to minimize the resistance across the silver, carbon, and primary cathode material layers in a solid electrolytic capacitor.
It is yet another object of this invention to improve the adhesive strength between the carbon layer and the primary cathode material in a solid electrolytic capacitor.
It is a still further object of this invention to produce a solid electrolytic capacitor with low ESR.
It is yet another object of this invention to minimize ESR shifts of a solid electrolytic capacitor during encapsulation and solder reflow.
Finally it is an object of the invention to avoid increase in ESR as a result of humidity during cleaning after mounting and when used in humid climates.
These and other objectives are obtained when a conductive polymer is applied as the first cathodic layer, followed by application of a carbon layer which is then infused with a conductive polymer before being coated with a metal paint prior to connection to a metallic terminal. Intrinsically conductive polymers are preferred for cathode and infusion with carbon and silver is the preferred metal for the metallic paint.